In many cellular radio systems, the cells are distinguished from a user equipment (UE) point of view by their cell identity numbers (cell-ids). A UE can be, but is not limited to, a mobile device. Different cell-ids often reflect different (primarily physical layer) transmission characteristics, which include but are not limited to, reference signal sequences, scrambling sequences, and synchronization signal properties. A UE can detect the cell-id of the cell it is connecting to based on certain estimated physical layer parameters and can subsequently learn other (e.g., primarily physical layer) properties for the detected cell-id. Each cell-id can be divided into multiple parts, of which one part can be detected more easily from certain physical layer parameters while another part can be read from broadcasted information.
A UE can often detect the identities of multiple nearby cells. Typically, a UE tries to connect to the cell that it identifies as the strongest in terms of transmission signal strength. In most cases, a UE is connected to only one cell at a time. When a UE is connected to a cell, the UE uses the transmission characteristics and parameters corresponding to the cell-id of the cell. In some cellular systems, a UE may connect to multiple cells at the same time, for example, during a hand-over procedure that changes cell to which the UE is connected.
An antenna serves a cell by transmitting and receiving signals related to the cell-id of the cell via the antenna. In a traditional cellular system implementation, the set of antennas serving a cell are quasi co-located, meaning that the set of antennas are roughly in the same location even if they may not occupy exactly the same physical space. One example of antenna quasi co-location is described in section 6.2.1 of 3GPP TS 36.211 V11.4.0 (2013-09), which is incorporated herein by reference. For a non-limiting example, antennas located on the same tower separated by a few meters are quasi co-located. For another non-limiting example, antennas located on the same building wall separated by a few meters are also considered as quasi co-located. For another non-limiting example, antennas located on opposite sides of a building, on the other hand, are not quasi co-located. UEs can often assume that quasi co-located antennas share some common or similar properties, such as the Doppler shift, among them. In a traditional cellular system deployment, antennas that are not quasi co-located serve cells having different cell ids.
In some cellular systems, a set of antennas that are quasi co-located can serve multiple cells. In some traditional cellular systems, however, different cells are served by different antennas even if they are quasi co-located. One example of this is a tower with quasi co-located antennas pointing in different directions, with the antennas pointing in one direction serving one cell, and the antennas pointing in another direction serving another cell.
In another kind of cellular system deployment, the antennas used for a cell are not quasi co-located. For a non-limiting example, such system can be a distributed antenna system (DAS) or a soft cell as described in, for example, Parkvall et al., “Heterogeneous network deployments in LTE”, Ericsson review no 2, 2011. One example of a DAS deployment is a single cell which covers the indoors of a building with antennas distributed on different floors using long antenna feeders. One example of a soft cell deployment is a traditional high-power macro cell transmitter with antennas on a tower and a low power transmitter with street-level antennas within the coverage area of the macro cell, wherein the low power transmitter uses the same cell-id as the macro cell. One way to distinguish a DAS from a soft cell is that the distributed antennas in a DAS transmit the same signals, whereas the distributed antennas in a soft cell may transmit different signals, albeit corresponding to the same cell-id. Note that not only the antennas may be distributed in a DAS or a soft cell, other components such as radio units and baseband units may also be completely or partly distributed.
In some cellular systems, a UE can learn parameters and settings of the cellular network and the cell it connects to from common information blocks, which are periodically broadcasted by the cellular network. Such broadcasted common information applies to all UEs in connection with the cell, but the information is usually different for different cells. In some cellular systems, coherent communication is used for these common information blocks, meaning that a UE needs to receive a certain known reference (pilot) signal together with the broadcasted information blocks. The reference signal is referred to hereinafter as the common reference signal and the common broadcasted information blocks are referred to hereinafter as the broadcast channel. In some cellular systems, the common reference signal is transmitted from the same set of antennas as the broadcast channel. A UE also needs to know the relation between the mapping of the modulated symbols of the broadcast channel to the set of antennas and the mapping of the common reference signal symbols to the set of antennas. These typically linear mappings are often referred to as precoding. The precoding operation can also include antenna subset selection from the set of antennas used by the common reference signal.
The common reference signal used for the reception of the broadcast channel is often also used for other important functions, such as power measurements for UE mobility and channel quality estimation. Hence, the common reference signal as well as the broadcast channel need to be transmitted at all times (regularly and periodically) in order to provide coverage without interruption in traditional cellular communication systems.
In some cellular communication systems with variable system bandwidth, the information about the downlink and uplink system bandwidths of the cells are broadcasted. For a non-limiting example, the UEs need to know the downlink system bandwidth in order to perform measurements on the common reference signal.
UE-dedicated control and data coherent transmission to the UE can also typically be performed with the assistance of the common reference signal. Another way to communicate UE-dedicated control and data coherently is to use another reference signal than the common reference signal, hereinafter referred to as a dedicated reference signal. Typically, only the UE receiving the dedicated control or data uses the dedicated reference signal. One advantage of using a dedicated reference signal is that an arbitrary precoding could be used that is unknown to the receiving UE. The coherent reception could still work if the same unknown precoding is applied to both the dedicated reference signal and the information symbols. In this way, a precoding that is particularly suitable for the receiving UE can be selected by the transmitter, without the need to inform the receiving UE about this and the precoding operation is transparent to the receiving UE in this case. Note that the transmission parameters of the dedicated reference signal and the dedicated control or data transmission may or may not depend on the cell-id.
The description above relates to one carrier, since typically one carrier corresponds to one cell with its own cell-id. According to examples in which multiple carriers are aggregated into one aggregate sharing a cell-id, the description above is similarly applicable to the aggregate.
In a non-limiting example of a cellular system, long-term evolution (LTE), UEs can detect the LTE network by searching for the primary synchronization signals (PSS) and secondary synchronization signals (SSS), which are periodically transmitted by an LTE eNodeB (eNB). From the detection of PSS/SSS, the UE can also learn the cell identity number (cell-id). After detection of PSS/SSS, the UE can receive the Master Information Block (MIB), which is transmitted on the Physical Broadcast Channel (PBCH) and the set of System Information Blocks (SIBs), which are transmitted on the Physical Downlink Shared Channel (PDSCH). These blocks contain common information that is necessary for the UE to function in the LTE cell. For a non-limiting example, the downlink system bandwidth is signaled in the MIB and the uplink system bandwidth is signaled in the second SIB. These information blocks are periodically broadcasted in the cell. The MIB and the SIBs in LTE are examples of broadcast channels discussed above,
For the demodulation of the MIB and the SIBs, the UE also needs to receive the cell-specific reference signal (CRS) transmitted over the whole downlink system bandwidth. The MIB and the SIBs are transmitted using a precoding. The precoding of the MIB is not entirely known by the UE and can be accomplished in one of a number of ways. The UE can assume that the precoding that results in a successful reception of the MIB was used at the transmitter. The CRS in LTE is an example of a common reference signal discussed above. In LTE, the demodulation of downlink control and data symbols (by a UE) can be based on either CRS (often called transmission mode 1-6) or UE-specific reference signals, also referred to as demodulation RS or DM-RS (often called transmission mode 7-9). UE-specific RS or DM-RS in LTE is an example of dedicated reference signal discussed above.
For CRS-based communication, the information symbols are advantageously transmitted from a set of antennas, from which CRS are also transmitted. Furthermore, the relation between the precoding of the CRS and the precoding of the control and data symbols is signaled to the UE. Note that the precoding of the CRS are advantageously known a priori, since they are used for receiving the broadcasted information blocks. For DM-RS based communication, the relation between the precoding of the DM-RS symbols and the precoding of the information symbols is a priori known by the receiving UE, but the precoding itself is not known or needed by the receiving UE.
Some LTE systems also use the antenna ports. One antenna port can be mapped to a multitude of antennas. For example, the CRS and broadcast channel in LTE can transmitted through two antenna ports, but eight antennas. The mapping from antenna port to antenna is transparent to the UEs.
The UEs may have different capabilities. Some UEs may be capable of downlink dedicated data and control communication based only on common reference signals. Other UEs may be capable of downlink dedicated data and control communication based only on dedicated reference signals. A third kind of UE may be capable of both. In LTE, for example, some UEs are capable only of CRS-based downlink communication. Other UEs are also capable of DM-RS based downlink communication.